Greece has 2 festivals in this guide, spread across 2 cities. They run in June and September, and festival-day temperatures range from 27°C (81°F) to 32°C (90°F), with nights down to 17°C (63°F). That range is the reason there is no single "Greece festival outfit". The answer changes by month and by site.
1 of them runs in real heat (28°C or more), where loose natural fabrics beat coverage and a brimmed hat does more than any garment.
The ground matters as much as the sky. Every festival here is on city streets, which puts broken-in trainers or a low boot at the top of the list for most of these events. None of them have camping on site, so you are travelling in and out each day and the outfit has to survive the journey at both ends.
Stylistically the crowd here leans urban, rock, metal and streetwear. Release Athens is the largest of them at around 40,000 people, and it sets the tone the smaller ones react to. None of that is a dress code. It is what you will see, which is worth knowing if you would rather blend in or deliberately not.